Spectroscopic valence of cerium in cerium-lanthanum-iron compounds

In ${\mathrm{Ce}}_2$ ${\mathrm{Fe}}_{14}$B and ${\mathrm{Ce}}_2$ ${\mathrm{Fe}}_{17}$, cerium is mixed valent and nonmagnetic. Any structural or compositional modification of these intermetallics which induces cerium to become trivalent and develop a local 4f moment should result in compounds with enhanced magnetostrictions, magnetocrystalline anisotropies, and net magnetic moments. Due to the dependence of cerium valence on steric volume, one approach for manipulating the valence is to increase the cell volume of these phases by forming pseudoternaries or pseudobinaries of these compounds with lanthanum. In this work, we report the spectroscopic valence of cerium obtained from the cerium ${\mathrm{L}}_3$ edge of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{14}$B, ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{17}$, and selected hydrides and nitrides of these intermetallics. In ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{14}$B the average cerium valence is found to decrease only slightly with increasing lanthanum content. Analysis of the extended x-ray absorption fine structure of the cerium and lanthanum ${\mathrm{L}}_3$ edges from the ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{14}$B series is only consistent with cerium preferentially occupying the small 4f rare-earth site in the tetragonal ${\mathrm{Nd}}_2$ ${\mathrm{Fe}}_{14}$B structure. In ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{Fe}}_{17}$ the shift of cerium towards the trivalent state with decreasing x is more pronounced, but the shift is incomplete over the range of compositions investigated (x⩾1). Although these results do not exclude the possibility of trivalent cerium in some of these compounds, none of the compounds provided unambiguous evidence of trivalent cerium.